In recent decades, the frequency and severity of flood-related damages and disruptions have dramatically increased.
In a compelling commentary published in the American Geophysical Union journal Earth’s Future, researchers from the University of California, Irvine and the University of Bristol in the United Kingdom—where key figures also lead the U.K. flood risk intelligence firm Fathom—call on scientists to more accurately model these risks and caution against overly dramatized reporting of future risks in the news media.
They also emphasize the need for responsible reporting on future flood dangers by the media. The researchers specifically encourage the climate science community to turn away from an outdated approach to mapping flood hazards known as “bathtub modeling,” which assumes floods spread uniformly over landscapes like a level pool.
Although this method provides a simplistic way to visualize flood impacts in coastal regions, it often results in a misleading and overly simplified representation of flood risks. Instead, they advocate for the use of sophisticated dynamical modeling, which employs physics-based equations to create a more accurate and realistic understanding of flood hazards.
“Bathtub models can both overpredict and underpredict flooding,” said co-author Brett Sanders, UC Irvine Chancellor’s Professor of Civil & environmental Engineering. “One of the biggest causes of error is that bathtub models fail to accurately account for the systems in place to protect people and assets, including storm drains, levees, and pumping.”
He and his team – including Oliver Wing, chief scientific officer at Fathom and an honorary research fellow at the University of Bristol, and Paul Bates, a University of Bristol professor of hydrology and Fathom chairman – highlight significant limitations of bathtub modeling when assessing flood risk.
These models fail to adequately consider at least six critical factors: the attenuation effects of event dynamics and friction on how floods spread; tidal amplification caused by the resonance of ocean tides in coastal embayments; the role of flood defenses such as levees and floodwalls, which may be overtopped during extreme events yet still help control inland flooding; the shoaling of groundwater tables; the emergence of groundwater influenced by rising sea levels and shifting hydrologic budgets; and the pumping of groundwater in areas below sea level to combat inundation from rising groundwater.
Through a thorough review of existing literature on flood risk, the research team emphasizes how bathtub models consistently demonstrate reduced accuracy, as quantified by the critical success index (CSI). This index ranges from 0 to 1, where 1 indicates a perfect match with field measurements.
Unfortunately, the CSI for the bathtub models discussed in the literature falls persistently below 0.5, significantly under the 0.65 threshold deemed necessary by experts for models to be locally relevant and thereby yield actionable results in impact analyses.
“CSIs under 0.5 indicate that these models are worse than a random classification,” Wing said. “In other words, a chimpanzee has more skill than a bathtub model in delineating flood hazard areas.”
Research indicates that bathtub modeling is frequently utilized in brief yet impactful journal articles, capturing significant attention from media outlets. While these technical papers often acknowledge the inherent biases and uncertainties of bathtub modeling, the narratives conveyed to the public and policymakers—often illustrated with striking images of submerged urban areas—can be misleading and exaggerated.
“Accurate maps of areas at risk of flooding are of paramount importance for everyone from home and business owners to insurers, banks, and governments,” Bates said. “We all have a role to play in reducing flood losses, but it all starts with trustworthy information.”
To effectively involve communities at risk in adaptation efforts and to develop fair and efficient mitigation and adaptation strategies, trustworthy flood risk models are essential, according to Sanders. Relying on flawed models may result in maladaptive responses.
“Projections of flooding need to make sense to people, not only for building understanding of what’s at risk but also for deciding upon the investments and policies that will be made to manage it,” Sanders said. “In fact, numerous research papers have shown that residents within at-risk areas are unlikely to trust projections of future flooding if they don’t reflect their lived experiences. Research studies that oversimplify flooding and don’t represent real-world data pose a threat to transformative action.”
Journal reference:
- Brett F. Sanders, Oliver E. J. Wing, Paul D. Bates. Flooding is Not Like Filling a Bath. Earth’s Future, 2024; DOI: 10.1029/2024EF005164